Composition for H2 S removal

ABSTRACT

A process for removal of H 2  S from gas streams is described, the process being characterized by use of a novel iron chelate treating solution containing a specified ferric to ferrous chelate ratio, aqueous ammonia, and thiosulfate ion.

This is a continuation of application Ser. No. 381,332 filed Jul. 18,1989, which in turn is a division of Ser. No. 75,196, filed Jul. 16,1987, which in turn is a continuation of Ser. No. 769,195 filed Aug. 23,1985.

BACKGROUND OF THE INVENTION

The presence of significant quantities of H₂ S in various "sour"industrial gaseous streams poses a persistent problem. Although variousprocedures have been developed to remove and recover this contaminant,most such processes are deficient, for a variety of reasons.

In one cyclic method currently attracting attention, the sour gas iscontacted with an aqueous polyvalent metal chelate or complex reactantsystem to produce solid sulfur which is recovered either prior to orsubsequent to regeneration of the reactant. Preferred reactants are iron(III) complexes in which the iron (III) forms complexes with specifiedorganic acids and derivatives thereof.

One of the disadvantages of such systems heretofore has been theinability to maintain sufficiently high concentrations of the chelate orcomplex to achieve efficient operation. Without sufficiently high levelsof the complex, these processes are limited in their ability to handlestreams containing significant quantities of H₂ S. Again, thecirculation of large volumes of dilute solutions to handle even moderatelevels of H₂ S involves significant capital and energy costs, especiallyin high pressure applications. Finally, degradation or decomposition ofthe polyvalent metal chelates represents an important most in theprocess, as well as requiring measures for decomposition bleed orremoval and addition of fresh solution. Even in the case of chelatessuch as those of nitrilotriacetic acid, ligand decomposition, over aperiod of time, requires attention to prevent build-up of decompositionproducts and consequent loss of efficiency. The invention addressesthese problems, and provides a novel composition and process for theresolution thereof.

SUMMARY OF THE INVENTION

Accordingly, the invention relates to a process for removing H₂ S froman H₂ S-containing or sour gaseous stream, of the type described, inwhich ammonium chelate-aqueous ammonia-containing solution which isstabilized to prevent complex degradation is provided for oxidation ofthe H₂ S. The ammonium ferrous chelate produced has a higher solubility,and more concentrated solutions may be employed. It is an additionalaspect of the invention that sodium or potassium ions are to be excludedfrom the solution employed, at least to the extent that the solubilitylimits of the sodium or potassium salts of the ferrous nitrilotriaceticacid chelate is exceeded. To this end, it is a further feature of theinvention that pH adjustment, which is necessary from time to time, ismade by the addition to the solution of ammonium hydroxide or carbonate.The invention also provides a solution that contains a high total ironcontent that is stable to precipitation, and that contains a specifiedrange of ratios of ammonium ferric iron chelate to ammonium ferrous ironchelate. It has been determined that maintenance of a relatively highferrous chelate concentration in the system, particularly duringregeneration, inhibits ligand degradation. Ligand degradation is furtherinhibited by the presence of thiosulfate ion, preferably supplied asammonium thiosulfate. The solution may be regenerated by contacting theaqueous admixture with oxygen under conditions to convert ammoniumferrous nitrilotriacetate chelate to ammonium ferric nitrilotriacetatechelate, producing regenerated aqueous reactant solution having a ratioof ammonium ferric nitrilotriacetate chelate to ammonium ferrousnitrilotriacetate chelate of from about 0.5 to about 6. To preventsulfur build up, sulfur is removed or recovered at a suitable locationin the system. Preferably, the sulfur is removed from at least the bulkof the admixture after removal of admixture from the contacting zone,but other suitable sites or loci, including slip streams from locationsin the process, e.g., from the contactor, may be utilized. The sulfurmay also be removed in a separate step after regeneration.

In a preferred form, the invention comprises a process for the removalof H₂ S from a sour gaseous stream comprising

contacting the sour gaseous stream with aqueous reactant solution in acontacting zone at a temperature below the melting point of sulfur, thereactant solution containing ammonium ferric nitrilotriacetate chelate,ammonium ferrous nitrilotriacetate chelate, aqueous ammonia, the ratioof the ammonium ferric nitrilotriacetate chelate to ammonium ferrousnitrilotriacetate chelate being from about 0.2 to about 6, preferablyabout 0.5 to about 6, and thiosulfate ion, the thiosulfate ion beingpresent in an amount sufficient to inhibit degradation, and having a pHof from about 5 to about 8.5, under conditions to convert H₂ S,producing a gaseous stream having reduced H₂ S content, and aqueousadmixture containing solid sulfur and additional ammonium ferrousnitrilotriacetate chelate in solution. Preferably, the aqueous admixtureis removed from the contacting zone and sulfur is removed from at leasta portion of said admixture. The aqueous admixture may then beregenerated by contacting aqueous admixture with oxygen in aregeneration zone under conditions to convert ammonium ferrousnitrilotriacetate chelate to ammonium ferric nitrilotriacetate chelate,producing regenerated solution having a ratio of ammonium ferricnitrilotriacetate chelate to ammonium ferrous nitrilotriacetate of fromabout 0.5 to 6. The regenerated aqueous reactant solution is then passedto the contacting zone for use as aqueous reactant solution therein.

In another embodiment, the invention relates to a process for theremoval of H₂ S from a sour gaseous stream comprising

a) contacting the sour gaseous stream with aqueous reactant solution ina contacting zone at a temperature below the melting point of sulfur,the reactant solution containing ammonium ferric nitrilotriacetatechelate, ammonium ferrous nitrilotriacetate chelate, aqueous ammonia,the ratio of the ammonium ferric nitrilotriacetate chelate to ammoniumferrous nitrilotriacetate chelate being from about 0.2 to about 6, andthiosulfate ion, the thiosulfate ion being present in an amountsufficient to inhibit degradation, and having a pH of from about 5 toabout 8.5, under conditions to convert H₂ S, producing a gaseous streamhaving reduced H₂ S content, and aqueous admixture containing solidsulfur and additional ammonium ferrous nitrilotriacetate chelate insolution;

b) removing aqueous admixture from the contacting zone and regeneratingaqueous admixture by contacting said aqueous admixture with oxygen in aregeneration zone under conditions to convert ammonium ferrousnitrilotriacetate chelate to ammonium ferric nitrilotriacetate chelate,and producing a regenerated aqueous reactant solution containing sulfurand having a ratio of ammonium ferric nitrilotriacetate chelate toammonium ferrous nitrilotriacetate chelate of from 0.5 to 6;

c) removing regenerated aqueous reactant solution from the regenerationzone and removing sulfur from at least a portion of the regeneratedsolution; and

d) passing regenerated reactant solution to the contacting zone for useas aqueous reactant solution therein.

As indicated, the invention further includes an aqueous solutioncomprising ammonium ferric nitrilotriacetate chelate, ammonium ferrousnitrilotriacetate chelate, aqueous ammonia, the ratio of ammonium ferricnitrilotriacetate chelate to ammonium ferrous nitrilotriacetate chelatebeing from about 0.2 to about 6, and ammonium thiosulfate, the ammoniumthiosulfate being present in a ratio of 0.01 mol to 4 mol of ammoniumthiosulfate, preferably 0.1 to 0.95 mol, per gram equivalent of ironpresent, based on the total concentration of ammonium ferricnitrilotriacetate chelate and ammonium ferrous nitrilotriacetatechelate, the solution having a pH of from about 5 to about 8.5. Thesolution may be synthesized, as described further herein, or may beproduced with the ferric-ferrous ratios specified, in the operations ofthe process of the invention. The total iron concentration in thesolution, as the chelates, based on the total amount of iron suppliedoriginally, will range from about 0.5 percent, preferably about 1% toabout 7% by weight, based on the weight of the solution and iron. In asolution that has been used in the removal of H₂ S from a gaseousstream, the total concentration of iron is not revealed by the sum ofthe concentrations of the ammonium ferric nitrilotriacetate chelate andthe ammonium ferrous nitrilotriacetate chelate, since other iron complexor chelate species are present. It has been determined, for example,that some degradation products of the chelates employed (and degradationproducts thereof) are soluble iron chelates. Again, there is evidence,for example, that the ammonium ferric nitrilotriacetate chelate may bepresent as a dimer. As used herein, the term "ammonium ferrousnitrilotriacetate chelate" refers to those solubilized species, which,upon precipitation, are believed to have the formula [NH₄ ][(NTA)Fe(H₂O)₂ ] where NTA refers to nitrilotriacetate. Because of the complexityof the system, however, and the difficulty of analysis of thecomponents, the term may also be considered as simply defining thespecies in the solution in which ammonium ion and ferrousnitrilotriacetate chelate are chemically associated or related, whateverthe precise nature or character of the relationship or bonding. The term"ammonium ferric nitrilotriacetate chelate" refers, correspondingly, tothe ferric species in the solution. The maximum solubility limit of theammonium ferrous nitrilotriacetate chelate is about 0.5M in unusedsolution.

As noted, the solutions employed will contain aqueous ammonia. As usedherein, the term "aqueous ammonia" is understood to include dissolvedammonia, ammonium hydroxide, and ammonium ion, as understood fromAdvanced Inorganic Chemistry, 3rd edition; F. A. Cotton and G.Wilkinson, especially page 349. The aqueous ammonia is present inaddition to the ammonium ion associated with or combined with the ferricnitrilotriacetate and ferrous nitrilotriacetate chelates. The quantitywill vary, but will be at least about 0.1 percent, on a molar basis,with respect to the total quantity of the ammonium ferric and ammoniumferrous chelates used. Preferably, the aqueous ammonia will be presentfrom, for example, 0.1 percent, to 200 or 300 percent, or more, on amolar basis, with respect to the quantity of the ammonium ferric andammonium ferrous nitrilotriacetate chelates present. In practice, thisquantity may be achieved by adjusting the pH of the solution withammonium hydroxide or carbonate to the levels described hereinafter.Ammonium compounds having salt forming anions, with the carbonateexception, are not desirable.

The presence of significant concentrations of sodium or potassium ionsis not desired in the solutions of the invention. The sodium andpotassium salts of the ferrous chelate of nitrilotriacetic acid havebeen determined to have undesirably low solubilities, so that they areunsuitable for high concentration operations. By avoiding thesematerials, the invention provides efficiencies not attainable with priorart processes. For example, since concentrations of the complexes of theinvention are higher, streams having greater concentrations of H₂ S maybe treated, at similar throughput. Pumping costs are reduced, and vesselsizes, other factors being equal, may be reduced. As noted, some sodiumor potassium ions are tolerable, provided that they are not present insufficient amount to exceed the solubility of sodium or potassiumferrous nitrilotriacetate chelate. In this regard, minor quantities ofsodium or potassium containing additives or buffers may be present.

For this reason, pH adjustment in the process of the invention isaccomplished by the use of ammonium hydroxide or carbonate. Prior artpractice of utilization of sodium hydroxide, potassium hydroxide, orsodium or potassium carbonates, is generally unsuited to the invention.The pH in the contacting zone will preferably be maintained in a rangeof from about 5 to 8.5, preferably 6.5 to 8.5, and in the regenerator,from about 7 to 8.5.

As stated, it has been found that minor amounts of thiosulfate ion,preferably supplied as ammonium thiosulfate, are useful in inhibitingdegradation of ammonium iron nitrilotriacetate chelates. The combinationof components, parameters and steps disclosed herein provides a processin which ligand degradation rates may be lowered to a degree believedpreviously unattainable. The thiosulfate may be supplied as the alkalimetal salt, e.g., lithium, sodium or potassium, provided the solubilityconsiderations noted in regard to the ammonium ferrous nitrilotriacetateare taken into account. The thiosulfate ion is supplied in an amountsufficient to inhibit degradation. Generally, the thiosulfate ion may besupplied in a concentration of about 0.2 to 2.5 molar, preferably about0.1 to 0.95 molar. The ion should be supplied in a ratio of about 0.01to 4 gram equivalents, preferably 0.1 to 0.95 gram equivalents, ofthiosulfate ion per gram equivalent of iron present, based on the totalconcentration of ammonium ferric nitrilotriacetate chelate and ammoniumferrous nitrilotriacetate chelate.

As noted, the regeneration of the reactant is accomplished by theutilization of oxygen, preferably as air. The oxygen will accomplish twofunctions, the oxidation of ferrous iron of the reactant to the ferricstate, and the stripping of any dissolved gas from the admixture. Theoxygen (in whatever form supplied) is supplied in a stoichiometricequivalent or excess with respect to the amount of ammonium ferrousnitrilotriacetic acid complex present in the mixture. Preferably, theoxygen is supplied in an amount of from about 20 percent to about 300percent excess.

As used herein, the term "oxygen" is not limited to "pure" oxygen, butincludes air, air-enriched with oxygen, or other oxygen-containinggases.

The particular type of sour gaseous stream treated is not critical, theonly practical limitation being the reactivity of the stream itself withthe solution employed, as will be evident to those skilled in the art.Streams particularly suited to removal of H₂ S by the practice of theinvention are, as indicated, naturally-occurring gases, recycled CO₂used in enhanced oil recovery, synthesis gases, process gases, and fuelgases produced by gasification procedures, e.g., gases produced by thegasification of coal, petroleum, shale, tar sands, etc. Particularlypreferred are coal gasification streams, natural gas streams, producedand recycled CO₂ streams, and refinery feedstocks composed of gaseoushydrocarbon streams, especially those streams of this type having a lowratio of H₂ S to CO₂, and other gaseous hydrocarbon streams. The term"hydrocarbon stream(s)", as employed herein, is intended to includestreams containing significant quantities of hydrocarbon (bothparaffinic and aromatic), it being recognized that such streams containsignificant "impurities" not technically defined as a hydrocarbon.Again, streams containing principally a single hydrocarbon, e.g.,ethane, are eminently suited to the practice of the invention. Streamsderived from the gasification and/or partial oxidation of gaseous orliquid hydrocarbon may be treated by the invention. The H₂ S content ofthe type of streams contemplated will vary extensively, but, in general,will range from about 0.005 percent to about 10 percent by volume. CO₂content will also vary, but may range from about 0.1 percent to about99.0 percent (or more) by volume. In this context, the invention may beused to remove H₂ S from various CO₂ streams, e.g., supercritical CO₂streams.

The temperatures employed in the contacting or absorption-contact zoneare not generally critical, except that the reaction is carried outbelow the melting point of sulfur. In many commerical applications, suchas removal of H₂ S from natural gas to meet pipeline specifications,absorption at ambient temperatures is desired, since the cost ofrefrigeration would exceed the benefits obtained due to increasedabsorption at the lower temperature. In general, temperatures of from10° C. to 80° C. are suitable, and temperatures of from 20° C. to 60° C.are preferred. Total contact times will range from about 1 second toabout 120 seconds, with contact times of 2 seconds to 60 seconds beingpreferred.

Similarly, in the regeneration or stripping zone or zones, temperaturesmay be varied widely. Preferably, the regeneration zone should bemaintained at substantially the same temperature, or somewhat lower, asthe contacting zone. In general, temperatures of from about 10° C. to80° C., preferably 20° C. to 50° C. may be employed.

Pressure conditions in the contacting zone may vary widely, depending onthe pressure of the gas to be treated. For example, pressures in thecontacting zone may vary from one atmosphere up to one hundred fifty oreven two hundred atmospheres. Pressures of from one atmosphere to aboutone hundred atmospheres are preferred. In the regeneration zone,pressures may also be varied considerably, and will preferably rangefrom about 1 atmosphere to about three or four atmospheres. Residencetimes for given volumes of admixture and oxygen will range from 10minutes to 60 minutes, preferably from 20 minutes to 40 minutes. Thepressure, fluid flow, and temperature relationships involved are wellunderstood by those skilled in the art, and need not be detailed herein.Other conditions of operation for this type of reaction process arefurther described in U.S. Pat. No. 3,068,065 to Hartley et al, datedDec. 11, 1962, incorporated herein by reference. Preferably, the molarratio of the nitrilotriacetic acid to total iron is from about 1.0 to1.5. The process is preferably conducted continuously.

As indicated, the H₂ S, when contacted, is rapidly converted in theprocess of the invention by the ammonium ferric nitrilotriacetatechelate to solid elemental sulfur. The amount of ammonium ferricnitrilotriacetate supplied or employed in solution is an amountsufficient to reduce the H₂ S concentration or content in the gaseousstream to the desired level. If total or substantially total removal isdesired, the total amount supplied will generally be on the order of atleast about two mols per mol of H₂ S. Ratios of from about 2 mols toabout 15 mols of ammonium ferric nitrilotriacetate chelate per mol of H₂S may be used with ratios from about 2 mols per mol to about 5 mols ofammonium ferric chelate per mol of H₂ S being preferred. As noted, theratio of ammonium ferric nitrilotriacetate chelate to ammonium ferrousnitrilotriacetate chelate present in solution will normally be less thanabout 6, and will normally range from about 0.2 to about 6, preferablyabout 0.5 to about 6.

The manner of preparing the solutions of the invention is, to someextent, a matter of choice. For example, the solutions employed in theprocess of the invention may be prepared by reaction of elemental ironwith nitrilotriacetic acid, as described in the U.S. Pat. No. 3,115,511,followed by air oxidation, pH adjustment with ammonium hydroxide,addition of the appropriate thiosulfate, e.g., ammonium thiosulfate, andappropriate water dilution to achieve the desired concentration.Alternately, nitrilotriacetic acid, ferrous carbonate ammonium hydroxideand oxygen (air) may be reacted to prepare the solution, with theaddition of the thiosulfate. The novel compositions of the invention maybe produced, for example, by separate reduction of one of the abovementioned solutions until the appropriate levels of the ammonium ferrouschelate are formed, or the solutions mentioned above may simply beemployed in the process of the invention until the appropriate ratios offerric to ferrous ligand are reached.

DETAILED DESCRIPTION OF THE INVENTION

In order to describe the invention in greater detail, reference is madeto the accompanying schematic drawing.

FIG. 1 of the drawing illustrates the embodiment of the inventionwherein sulfur removal is accomplished in a separate step prior toregeneration, while

FIG. 2 illustrates the case where sulfur is removed in a separate stepafter regeneration. All values are calculated or merely exemplary, andall flows, unless stated otherwise, are continuous.

As shown, sour gas, e.g., natural gas containing about 0.5 percent H₂ S,in line (1) enters contactor or column (2) into which also enters anaqueous mixture comprising an aqueous 0.8M solution of ammonium ferricnitrilotriacetate chelate, which mixture also contains 0.2 moles perliter of the ammonium ferrous nitrilotriacetate chelate and 0.3 molesper liter of ammonium thiosulfate. The solution is produced byutilization of the reducing effect of the H₂ S in the gaseous stream.That is, the initial solution in the contactor contains the thiosulfateand is a 1M aqueous solution of ferric nitrilotriacetate with a totalconcentration of ammonium ion and aqueous ammonia of 3 Molar. Afterstartup, and reaction with the H₂ S in the gaseous stream, regeneration,described hereinafter, is controlled so that regeneration of theammonium ferric nitrilotriacetic acid complex is not complete, in theratios mentioned. Absorber or contactor (2) may be of any suitable type,such as a packed column or tray column, but is preferably a combinationventurispray column system as described in commonly assigned, copendingapplication Ser. No. 769,199, entitled "Staged Removal of H₂ S from GasStreams", filed even date herewith, incorporated herein by reference.Depending on the size of the gas stream, the H₂ S content, etc., morethan one contactor unit may be employed, preferably in series. In anyevent, in the unit illustrated, the pressure of the feed gas is about1200 p.s.i.g., and the temperature of the aqueous mixture is about 45°C. A contact time of about 120 seconds is employed in order to react allthe H₂ S. Purified or "sweet" gas leaves column (2) L through line (3).The "sweet" gas is of a purity sufficient to meet standard requirements.In the mixture, the H₂ S is converted to solid elemental sulfur by theammonium ferric nitrilotriacetate chelate, ammonium ferricnitrilotriacetate chelate in the process being converted to ammoniumferrous nitrilotriacetate chelate. The aqueous admixture produced,containing elemental sulfur, is removed continuously, and sent throughline (4) to a depressurization and degassing unit (5), which also servesas a sulfur concentration or thickening zone. A minor portion, e.g., 5to 10 percent by volume of the admixture in thickener (5), andcontaining an increased sulfur concentration, is continuously withdrawnfrom the lower portion of concentrator (5) and sent via line (6) tosulfur recovery.

Sulfur recovery may be accomplished in any suitable fashion, such as byfiltration. Preferably, however, sulfur is recovered by that methoddescribed in commonly assigned, copending application Ser. No. 769,198entitled "Separation of Sulfur from Chelate Solutions", filed even dateherewith, incorporated herein by reference. Solution recovered duringsulfur recovery may be returned to any suitable point in the process, ifproper adjustment is made, but is preferably sent, as shown, to theregeneration zone. The major portion of the aqueous admixture in vessel(5), having a reduced sulfur content, is removed via line (7) forregeneration of ammonium ferric nitrilotriacetate chelate. Inregeneration zone or column (8), the admixture L is contacted withexcess air from line (9) to convert the ammonium ferrousnitrilotriacetate chelate to ammonium ferric nitrilotriacetate chelate.

Regeneration zone (8) comprises a sparged tower regenerator withcocurrent upflow of oxygen (as air), via line (9), and aqueousadmixture. Air velocity in the regenerator is in the range of 0.1 to 0.3feet per second. The temperature in the column is about 45° C., andoverall pressure is about 2 atmospheres. Spent air is removed via line(11), and regenerated admixture, having a ratio of ammonium ferricnitrilotriacetate chelate to ammonium ferrous nitrilotriacetate chelateof about 4 is returned via line (12) to column 2.

In FIG. 2, sour gas e.g., natural gas containing about 0.5 percent H₂ S,and 32 percent by volume CO₂, in line (31) enters column (32) (spargedcolumn type) and contacts an aqueous 0.8M solution of ammonium ferricnitrilotriacetic chelate, also containing 0.2 moles per liter ammoniumferrous nitrilotriacetate chelate and 0.3 moles per liter of ammoniumthiosulfate, the total concentration of ammonium ion and aqueous ammoniabeing 3 Molar. The pressure of the feed gas is about 1200 p.s.i.g., andthe temperature of the aqueous mixture is about 45° C. A contact time ofabout 45 seconds is employed in order to react all the H₂ S. Purified or"sweet" gas leaves column (32) through line (33). The "sweet" gas is ofa purity sufficient to meet standard requirements. In the aqueousmixture, the H₂ S is converted to elemental sulfur by the solubilizedferric chelate. The aqueous mixture, containing elemental sulfur, aslight amount of absorbed CO₂, and about 0.5 moles per liter solublizedferrous chelate, is removed continuously and sent through line (34) todegassing. As shown, any dissolved gases are removed in unit (35) byreduction of pressure, and the admixture forwarded via line (36) tocolumn (37).

In regeneration zone (37), admixture is treated in a similar fashion tothat described with reference to FIG. 1. Ammonium ferrousnitrilotriacetate chelate is converted by oxygen (line 38) to theammonium ferric chelate, while maintaining sufficient ammonium ferrousnitrilotriacetate chelate to inhibit degradation of the iron chelates.The temperature of the column (37) is about 45° C., and the pressure inthe column is maintained at about 2 atmospheres. The regeneratedadmixture, which still contains elemental sulfur, and spent excess airis sent through line (39) to degassing and thickening zone (40). Spentair is removed from column (40) through line (41). From unit (40), whichcorresponds to concentrator or separator (5), major and minor portionsof the regenerated, solid sulfur-containing admixture are separated, themajor portion being sent through line (42) to the contactor (32). Theminor portion, e.g., 5 percent by volume of the mixture in (40), andcontaining an increased sulfur L content, is sent through line (43) tosulfur recovery in the manner described in relation to FIG. 1. Admixturefrom sulfur recovery is returned to the system via line (42).

While the invention has been illustrated with particular apparatus,those skilled in the art will appreciate that, except where specified,other equivalent or analogous units may be employed. The term "zones",as employed in the specification and claims, includes, where suitable,the use of segmented equipment operated in series, or the division ofone unit into multiple units because of size constraints, etc. Forexample, a contacting zone may comprise two separate countercurrentcolumns in which the solution from the lower portion of the first columnwould be introduced into the upper portion of the second column, thepartially purified gaseous material produced from the upper portion ofthe first column being fed into the lower portion of the second column.Parallel operation of units, is, of course, well within the scope of theinvention. Admixture or solution withdrawal or introduction may be madeat any suitable site(s) or loci in the particular zone. The return ofsolution to one or more multiple contacting units in the contacting zonefrom a regenerator, or the use of a common regenerator, is within thescope of the invention.

Again, as will be understood by those skilled in the art, the solutionsor mixtures employed may contain other materials or additives for givenpurposes. For example, U.S. Pat. No. 3,933,993 discloses the use ofbuffering agents, such as phosphate and carbonate buffers. Similarly,other additives, such as antifoaming and/or wetting agents, may beemployed.

What is claimed is:
 1. A composition of matter comprising an aqueoussolution comprising ammonium ferric nitrilotriacetate chelate andammonium ferrous nitrilotriacetate chelate, the molar ratio of ammoniumferric nitrilotriacetate chelate to ammonium ferrous nitrilotriacetatechelate being from about 0.2 to about 6, at least about 0.1 percent,molar basis, with respect to said ammonium ferric and ammonium ferrousnitrilotriacetate chelates, of aqueous ammonia, and ammonium thiosulfatein a ratio of 0.01 mol of about 4 mols of ammonium thiosulfate per gramequivalent of iron present, based on the total concentration of ammoniumferric nitrilotriacetate chelate and ammonium ferrous nitrilotriacetatechelate, the solution being a pH of from about 5 to about 8.5, the totaliron content of the solution being from about 0.5 to about 7 percent byweight, based on the weight of the iron and the solution, and the totalconcentration of sodium or potassium ions present being less than thatsufficient to precipitate sodium or potassium ferrous nitrilotriaceticacid.